FIG. 3 is a cross-sectional view of a semiconductor laser including an active layer of a stress compensation type multiple quantum well structure, disclosed in, for example, Applied Physics Letter, Vol. 62, No.14 (1993), p.1644.
The semiconductor laser shown in FIG. 3 emits laser light of wavelength 0.98.about.1.02 .mu.m, and includes an active layer in which plural well layers and plural barrier layers are alternatingly laminated. In the 0.98.about.1.02 .mu.m band semiconductor laser, the lattice constant of a crystalline semiconductor constituting the well layers must be larger than the lattice constant of a crystalline semiconductor constituting the substrate. However, when there is no lattice-match between the crystalline semiconductors, defects tend to occur due to a stress generated between the crystalline semiconductors. Therefore, the stress generated between the crystalline semiconductors is offset by making the lattice constant of the crystalline semiconductor constituting the barrier layers smaller than that of the crystalline semiconductor constituting the substrate.
To be specific, the semiconductor laser 1 shown in FIG. 3 comprises an n type GaInP lower cladding layer 3, an undoped GaInAsP first light confinement layer 4, an active layer 15, an undoped GaInAsP second light confinement layer 12, a p type GaInP upper cladding layer 13, and a p type GaAs contact layer 14, which are successively disposed on an n type GaAs substrate 2. The active layer 15 comprises an undoped GaAsP first barrier layer 5, an undoped InGaAs first well layer 6, an undoped GaAsP second barrier layer 7, an undoped InGaAs second well layer 8, an undoped GaAsP third barrier layer 9, an undoped InGaAs third well layer 10, and an undoped GaAsP fourth barrier layer 11.
In has an effect of increasing the lattice constant of GaAs, and P has an effect of reducing the lattice constant of GaAs. Using the properties of In and P, the lattice constant of each well layer comprising InGaAs is made larger than that of the substrate 2 comprising GaAs, and the lattice constant of each barrier layer comprising GaAsP is made smaller than that of the substrate 2 comprising GaAs. Thus, the stress generated between the crystalline semiconductors is offset to provide an active layer 15 having required stress compensation performance.
When the respective semiconductor layers described above are grown on the substrate 2, for example, MOCVD (Metal Organic Chemical Vapor Deposition) is employed. However, the following problems arise in the MOCVD growth when switching gases for crystal growth supplied to a reaction furnace.
When the well layers 6, 8, and 10 comprising InGaAs and the barrier layers 5, 7, 9, and 11 comprising GaAsP are alternatingly grown, source gases of these materials should be switched. However, quick switching between the source gas for As (AsH.sub.3) and the source gas for P (PH.sub.3) is particularly difficult.
This problem will be described in more detail. The composition ratio of As in InGaAs constituting the well layers 6, 8, and 10 is different from the composition ratio of As in GaAsP constituting the barrier layers 5, 7, 9, and 11. In addition, the barrier layers 5, 7, 9, and 11 contain P whereas the well layers 6,8,and 10 do not contain P. Therefore, when the well layers 6, 8, and 10 and the barrier layers 5, 7, 9, and 11 are alternatingly grown, switching of the supply amount of AsH.sub.3 gas and switching between suplying and stopping of PH.sub.3 gas are performed.
On the other hand, group V gases, such as AsH.sub.3 gas and PH.sub.3 gas, tend to remain in the reaction furnace and, therefore, reliable switching of the group V gas is not easy. Besides, if the switching of the group V gas is not carried out reliably, desired lattice constants of the well layers 6, 8, and 10 and the barrier layers 5, 7, 9, and 11 are not realized. As a result, an active layer 15 having expected stress compensation performance is not produced. That is, in the prior art laser mentioned above, it is difficult to stably produce the above-described active layer 15.